In general, blow molding is a method in which preliminary molding is performed by using extrusion or injection to form a tube, this is provided in a mold, air is blown thereinto to swell the resulting structure, and cooling solidification is performed to obtain a molded body having a predetermined shape. The blow molding is largely classified into extrusion blow molding (extrusion or direct blow molding), injection blow molding, and stretch blow molding according to a preliminary molding method.
Hollow bottle products having a small thickness and various sizes, which contains liquid substances such as a liquid soap, a bleaching agent, an antifreezing solution, an engine oil, cosmetics, and medicines, are manufactured by using the blow molding. In the manufacturing of them, the high density polyethylene is mainly used, but the low density polyethylene may be used to manufacture a squeeze bottle. In addition, the high density polyethylene having the very high molecular weight is used to manufacture middle- and large-sized containers such as a soy sauce bottle, a mineral water bottle, chemical bottle and the like, and an ultra large-sized drum can.
In respects to the resin that is used in the blow molding, since a melt preliminary molded product (Parison) descends to a predetermined level while having a predetermined tension, it is required that the product has a predetermined melt tension in a melt state. In addition, since the high production rate reduces the cost, it is required that the melt index is low. In general, polyethylene having the melt index of 1.0 or less is used. In addition, it is required that the resin that is used to manufacture the chemical tub and the like has high chemical stability and environmental stress cracking resistance (ESCR).
Polyethylene is extensively used in order to manufacture molded products having various sizes. The reason for this is that polyethylene has excellent mechanical strength, full notch creep test, and chemical resistance and has a light weight.
Korean Patent Application No. 2000-0048952 discloses a linear low density polyethylene resin for blow molding which is polymerized by using a Ziegler catalyst and excellent impact resistance to falling and a desirable appearance, thus being used to manufacture food vessels such as soft bottles for mayonnaise, and a blow molded material manufactured by using the resin. However, in general, linear low density polyethylene has poor distortion strength due to low density and has a limit in application to products due to the frequent occurrence of sharkskin in the used bottles.
In general, high density polyethylene is extensively used to manufacture bottles for various purposes. The molecular weight distribution may be defined by a curve obtained from a gel permeation chromatography, and general high density polyethylene has very narrow molecular weight distribution of 8 or less. If a plastic vessel is manufactured by using the resin having the narrow molecular weight distribution, the manufactured vessel has very high luster but it is very difficult to process it. In addition, the resin having the narrow molecular weight distribution has poor mechanical properties, in particular, very low environmental stress cracking resistance (Modern Plastic International, August 1993, p 45). The molecular weight refers to weight average molecular weight herein.
U.S. Pat. No. 6,180,736 discloses a method of manufacturing polyethylene in a single gas phase reactor or a continuous slurry reactor by using one type of metallocene catalyst. When this method is used, there are advantages in that the production cost of polyethylene is low, fouling does not occur, and polymerization activity is stable. In addition, U.S. Pat. No. 6,911,508 discloses the manufacturing of polyethylene that is polymerized in a single gas phase reactor and has improved rheological properties by using a novel metallocene catalyst compound and 1-hexene as a comonomer. However, since polyethylene that is generated in the above-mentioned patents has a narrow molecular weight distribution, there are disadvantages in that it is difficult to ensure sufficient impact strength and processability.
U.S. Pat. No. 4,935,474 discloses a method of manufacturing polyethylene having a wide molecular weight distribution by using two or more metallocene compounds. In addition, U.S. Pat. Nos. 6,841,631 and 6,894,128 disclose that polyethylene having a bimodal or multimodal molecular weight distribution is prepared by using a metallocene catalyst containing at least two metal compounds, thus being used to manufacture films, pipes, and blow molded products. However, even though prepared polyethylene has improved processability, since the dispersion state according to the molecular weight in a unit particle is not uniform, an appearance is rough and physical properties are not stable under relatively desirable processing conditions.
Therefore, there is a need to manufacture an excellent resin in which a balance is ensured between physical properties or between physical properties and processability, and a demand for study regarding this is required.